Abstract: Described is a method for the treatment or prevention of GI tract dysbiosis in a subject, said method including administering an effective amount of a non-viable Methylococcus capsulatus or a lysate of Methylococcus capsulatus to said subject. The invention further provides a method for the treatment or prevention of a disease or condition selected from small intestine bacterial overgrowth (SIBO), small intestine fungal overgrowth syndrome (SIFO), GI tract cancers, breast cancer, neurological disorders, malnutrition, chronic fatigue syndrome, autism, cardiovascular diseases and GI tract infections in a subject with GI tract dysbiosis, said method including administering an effective amount of a non-viable Methylococcus capsulatus or a lysate of Methylococcus capsulatus to said subject. A non-viable Methylococcus capsulatus or a lysate of Methylococcus capsulatus for use in said methods is further provided.